Currently, blown films are made predominantly from ethylene polymers. There are references to blowing films of propylene polymers, but none are observed to be commercially successful. The low melt strength of propylene polymers inhibits production of blown film with such polymers at commercially feasible rates on standard equipment.
Scheve et al. in U.S. Pat. No. 5,519,785 disclosed the use of polypropylenes having a branching index less than one and having a strain hardening elongational viscosity to blow certain films. The polymers were treated with radiation under specified conditions in a multistep process which involves specialized equipment in steps after polymerization. Such a process is multi step, difficult and preferably avoided.
Giacobbe and Pufka in U.S. Pat. No. 5,641,848 disclose making blown films from a propylene polymer material of broad molecular weight distribution (MWD of about 4-60), a melt flow rate of about 0.5 to 50 dg/min. and xylene insolubles (at 25xc2x0 C.) of greater than or equal to 94 percent, said propylene polymer material selected from a broad molecular weight distribution propylene homopolymer and an ethylene propylene rubber impact modified broad molecular weight homopolymer. But this blend forms blown films at rates lower than those used commercially for polyethylene blown films.
In some instances, blowing films of polypropylene has been achieved by coextruding a polypropylene with another polymer. For instance, Nicola disclosed in DE 19650673 the use of a rubber modified polypropylene layer between polypropylene layers. Similarly, Landoni in EP 595252 disclosed the use of linear low density polyethylene (LLDPE) or linear medium density polyethylene, optionally with added hydrogenated hydrocarbon resins or other resins or low molecular weight polyethylene or polypropylene waxes between external layers of polypropylene. In EP 474376, Schirmer et al. disclose the use of an ethylene vinyl acetate copolymer (EVA), very low density polyethylene (VLDPE) or ethylene alpha olefin copolymer with a broad molecular weight distribution with a polypropylene layer and a sealable layer.
It would therefore be desirable to have a propylene polymer composition with sufficient melt strength to maintain a stable bubble for blown film manufacture on commercially available equipment, preferably that equipment available for the blowing of ethylene polymer compositions, more preferably both air and water quenched blown film equipment in both high and low stalk configurations, that is equipment commonly used for high density and low density polyethylenes (LDPE), respectively. The term xe2x80x9cstalkxe2x80x9d is used to designate the neck height of a bubble of polymer being blown into film. To achieve this end, a propylene polymer composition would advantageously have a melt strength that is higher than about 10, preferably between 10-100 cN, more preferably between 20-80 cN, and most preferably between 25-75 cN (measured at 190xc2x0 C.). Further, it is desirable that the resulting film shows at least a mechanical properties balance.
Rheology modification of the propylene polymers through reaction with coupling agents has now been found to improve the melt strength of the propylene polymers sufficiently to permit production of blown films (both coextruded and monolayer films) with the rheology modified propylene polymers at commercially acceptable rates.
As used herein, the term xe2x80x9crheology modificationxe2x80x9d means change in the resistance of the molten polymer to flow. The resistance of polymer melts to flow is indicated by (1) the tensile stress growth coefficient and (2) the dynamic shear viscosity coefficient. The tensile stress growth coefficient xcex7E+ is measured during start-up of uniaxial extensional flow by means within the skill in the art such as is described by J. Meissner in Proc. XIIth International Congress on Rheology, Quebec, Canada, August 1996, pages 7-10 and by J. Meissner and J. Hostettler, Rheol. Acta, 33, 1-21 (1994). The dynamic shear viscosity coefficient is measured with small-amplitude sinusoidal shear flow experiments by means within the skill in the art such as described by R. Hingmann and B. L. Marczinke, J. Rheol. 38(3), 573-87, 1994.
In one embodiment, the invention is a coextruded film comprising (a) at least one layer comprising at least one coupled propylene polymer coupled by reaction with a coupling agent; and (b) at least one layer comprising an in-reactor blend of a substantially linear polyethylene (or a homogeneously branched linear polyethylene) and a linear low density polyethylene. Component (a) may also contain LDPE, LLDPE, HDPE, substantially linear polyethylene, homogeneously branched linear polyethylene, and blends thereof. Preferably, component (a) contains LLDPE and/or substantially linear polyethylene (or homogeneously branched linear polyethylene) in order to improve the compatibility between component (a) and component (b). In some embodiments, it is more preferable that component (a) contain an in-reactor blend of a substantially linear polyethylene (or a homogeneously branched linear polyethylene) and a linear low density polyethylene.
Particular embodiments are those articles including an institutional liner, consumer liner, heavy duty shipping sack, produce bag, batch inclusion bag, pouch, grocery bag, merchandise bag, packaging, cereal liner, soft paper overwrap, multi-wall bag, lamination or combination thereof, including multiwall or multilayer configurations thereof.
All embodiments of the invention provide improved film processing characteristics (compared to films made without a coupled propylene polymer), and provide films exhibiting excellent mechanical properties, such as Elmendorf tear, 2% secant modulus, puncture resistance and Dart impact.